Metal working – Method of mechanical manufacture – Heat exchanger or boiler making
Utility Patent
1998-11-13
2001-01-02
Cuda Rosenbaum, I (Department: 3726)
Metal working
Method of mechanical manufacture
Heat exchanger or boiler making
C029S890047, C029S727000
Utility Patent
active
06167619
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved method for joining tubes to an array of fins for the purpose of assembling a heat exchanger. More particularly, this invention relates to an improved method for mechanically joining tubes and fins, in which a tube is deformed by longitudinal compression without intrusion into the tube passage, causing the tube to expand radially outward to engage the fins and any other hardware to be mounted to the tube.
2. Description of the Prior Art
Heat exchangers are widely used in various industries in the form of radiators for cooling motors, engines, and steering, transmission and hydraulic fluids, condensers and evaporators for use in air conditioning systems, and heaters. In their most simple form, heat exchangers include one or more passages through which a fluid flows while exchanging heat with the environment surrounding the passage. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and fluid, the design of a heat exchanger is typically of a tube-and-fin type containing a number of tubes that thermally communicate with fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. Various heat exchanger designs are known in the prior art. Design variations include the manner in which the fluid passage is constructed and the type of fin used. For example, the passage may be composed of one or more serpentine tubes that traverse the heat exchanger in a circuitous manner, or a number of discrete parallel tubes joined, typically brazed, to and between a pair of headers. The fins may be provided in the form of panels having apertures through which the tubes are inserted, or in the form of centers that can be positioned between adjacent pairs of tubes.
Conventionally, heat exchangers are manufactured by joining the tubes and fins using a brazing operation or a mechanical expansion technique. Mechanical expansion techniques rely solely on the mechanical joining of the components of the heat exchanger to ensure the integrity of the heat exchanger. Advantages of mechanical expansion techniques include good mechanical strength and avoidance of joining operations that require a furnace operation. The thermal performance of mechanically joined tubes and fins relies on adequate contact between the tubes and fins. Accordingly, improvements in mechanical expansion techniques have often been directed to ways in which the uniformity and integrity of the tube-to-fin joint can be improved. Conventional mechanical expansion methods can generally be categorized as being external or internal operations. Internal expansion techniques typically entail forcing an expansion tool, such as a mandrel or bullet, into the tubes, or by applying hydraulic internal pressure to the tubes. These methods physically force the walls of the tubes outward and into engagement with the fins. In contrast, external expansion techniques have generally entailed deforming the tubes with a tool that impacts or presses the tubes into engagement with the fins. While internal expansion methods tend to be characterized by enhanced joint strength and a lower resistance to heat transfer, the intrusion of a tool or fluid into the tubes is generally undesirable from the standpoint of the potential for introducing contaminants into the tubes, necessitating post-forming cleaning operations. Furthermore, prior art methods for deforming a tube wall raise the potential for excessive wall thinning, and therefore reduced strength. Finally, internal expansion methods are not well suited for use with heat exchangers formed with a serpentine tube. In contrast, external expansion methods generally cannot yield uniform tube-to-fin contact around the entire perimeter of a tube.
From the above, it can be appreciated that it would be advantageous if an improved method were available for mechanically joining the tubes and fins of a heat exchanger. Such a method would preferably result in joint strength comparable to internal expansion methods, but rely entirely on an external expansion technique so as to avoid the disadvantages of internal expansion methods, including the potential for contamination and wall thinning. A preferred technique would also be well suited for use on heat exchanger designs incorporating a serpentine tube configuration.
SUMMARY OF THE INVENTION
According to the present invention, a method is provided for assembling a heat exchanger unit that is suitable for use as a radiator for cooling a motor or engine, a condenser or evaporator for use in air conditioning systems, an oil cooler for power steering fluids, automatic and manual transmission fluids, after coolers for air and hydraulic system fluids, or a heater. The method involves a novel expansion technique that, without physical intrusion into the tube passage, produces a tube-to-fin joint that exhibits enhanced mechanical joint strength and metal-to-metal contact between the tubes and fins of a heat exchanger. Consequently, the method of this invention avoids the shortcomings of internal expansion techniques, and provides a significant improvement over prior art external expansion techniques.
The method of this invention generally includes forming a number of fins for assembly with one or more tubes having substantially parallel tube portions. Pairs of tubes portions may be connected by a bend or an elbow to yield a serpentine tube configuration. Each of the fins is formed to include one or more apertures for receiving each tube with which the fin is to be assembled. The fins are then arranged to form a fin pack, i.e., an array of substantially parallel fins, such that their apertures are aligned to form an aggregate passage through the fin pack. The tube portions are then inserted into the aggregate passage, such that the bend or elbow (if present) remains outside the fin pack. Finally, the tube portions are expanded to contact and become mechanically secured to their respective fins through the application of a force in a longitudinal direction to the tube portions. More specifically, the ends of the tube portions are fixtured and the longitudinal force applied through the fixtures, which causes the tube portions to bulge radially outward to create an interference fit between the tube portions and fins. Any brackets or other hardware intended to be joined to the tube can be simultaneously secured by the radial bulging of the tube portions.
Surprisingly, if the tubes are properly fixtured, deformation has been found to be uniform around the perimeter of each tube portion, so that a uniform interference fit is produced between each tube portion and its fins, thereby promoting heat transfer therebetween. Advantageously, the required longitudinal force can be readily controlled such that only the tube portions are deformed, with any bulging of the tube portions beyond that required to engage the fins and hardware being localized in regions of the tube portions between fins, which further promotes the structural integrity of the resulting tube-and-fin assembly. In that a compressive force is used, wall thinning does not occur in the tube portions. To the contrary, wall thickening may occur.
The above assembly method enables the insertion of the tube portions into the fin pack and the expansion of the tube portions to be performed in an uncomplicated operation. For some applications it is possible for the fixturing employed to insert a tube into a fin pack to also serve as the fixturing by which the longitudinal compressive force is applied to expand the tube. The method of this invention is greatly simplified in comparison to prior art assembly methods used to achieve comparable joint strength and integrity, such as internal expansion techniques and braze operations. Furthermore, the method of this invention can be employed to secure fins to a continuous serpentine tube, in which the tube portions and bend or elbow are part of an integrally-formed fluid p
Blissfield Manufacturing Company
Cuda Rosenbaum I
Hartman Domenica N. S.
Hartman Gary M.
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